Hexapeptide acyltransferases are characterized by tandem repeated copies of a six residue 'hexapeptide repeat' periodicity theme that encode folding of a unique left-handed parallel beta-helix structural domain. These enzymes play important roles in detoxification, cell wall formation and amino acid biosynthesis, but are absent in humans. Three bacterial hexapeptide acetyltransferases play well understood biological roles, but have not been mechanistically characterized or developed as antimicrobial drug targets. The gram-positive streptogramin acetyltransferase (VatD) inactivates the group A component of streptogramin mixtures currently used as last resort antibiotics in human medicine. High throughput screening and structure determination of enzyme-ligand complexes will be used to characterize the interaction of inhibitors with Vat(D). These compounds, or their synthetic variants, may find use as antibiotics that restore the susceptibility of gram positive pathogens to existing streptogramin mixtures. The bifunctional uridyltransferase (GImU) is responsible for the biosynthesis of UDP-GlcNAc, an essential building block for both the peptidoglycan and lipopolysaccharide components of bacterial cell walls. Its acetyltransferase active site is unique but has not been mechanistically characterized. Its structure and mechanism of action will be defined in order to develop the potential of GlmU as an attractive mechanism-based antibacterial target. Serine transacetylase (STA) O-acetylates serine in the cysteine biosynthetic pathway of bacteria and is the key regulatory enzyme of cysteine biosynthesis and sulfate assimilation. The structure and reaction mechanism of STA will be studied as a resolved enzyme and as a component of the binary cysteine synthase complex. STA is inhibited by cysteine, but must also utilize serine. The means by which STA recognizes these isosteric amino acids as inhibitor and substrate, respectively, may point to features of antimicrobial agents that duplicate this natural high-affinity mode of inhibition. The successful completion of these aims will serve to advance basic functional understanding of the hexapeptide acetyltransferase superfamily of enzymes and will develop their application as screening-based or mechanism-based targets for antibacterial drug design.
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